Generally, when an image includes a bright area (area having high luminance values) and a dark area (area having low luminance values), although there is a luminance difference between the two areas, each area has no luminance difference within itself, and contrast tends to be small which renders the image unclear. As a measure for dealing with this problem, a method is known which creates an image luminance histogram having a horizontal axis representing luminance value (tone value) and a vertical axis representing frequency (number of times), and enhances contrast of tones having a large number of elements (frequency of the same luminance value) by carrying out tone conversion which uniforms luminance distributions. Contrast can be enhanced by carrying out a tone correction in this way. Various techniques for tone correction have been proposed (see Patent Documents 1 and 2, for example). Applicant herein has proposed a technique for detecting a main inspection object area (e.g. a lung field area) for which contrast enhancement is desired (see Patent Document 3, for example).
A method providing an increased effect is also known, which calculates a luminance histogram for each of a plurality of areas obtained by dividing an image, instead of one histogram for the image, and carries out tone correction for each area. Patent Document 1: Japanese Unexamined Patent Publication No. 2011-130243 discloses a method which divides an image into a plurality of block areas, carries out contrast correction for each block area, and carries out a tone conversion that does not make brightness (luminance value) discontinuous at a boundary portion between two adjoining block areas.
Patent Document 1 (Japanese Unexamined Patent Publication No. 2011-130243; Patent Document 2 (Japanese Unexamined Patent Publication No. 2011-205714; Patent Document 3 (Japanese Unexamined Patent Publication No. 2011-255033)